The heat shock factor HSF1 juggles protein quality control and

JCB: Spotlight
Published February 9, 2017
The heat shock factor HSF1 juggles protein quality
control and metabolic regulation
Carles Cantó1,2
Institute of Health Sciences, Lausanne CH-1015, Switzerland
of Life Sciences, Ecole Polytechnique Fédérale de Lausanne, Lausanne CH-1015, Switzerland
Transcriptional regulators often act as central hubs to
integrate multiple nutrient and stress signals. In this issue,
Qiao et al. (2017. J. Cell Biol. https​://doi​.org​/10​.1083​/
jcb​.201607091) demonstrate how heat shock factor 1
(HSF1) uncouples metabolic control from proteostatic
regulation and unveils HSF1 as a critical transcriptional
regulator of NAD+ metabolism.
The ability to respond and adapt to environmental and cellular challenges is a key evolutionary driver for organisms across
all biological domains. Consequently, complex organisms, like
most eukaryotes, are endowed with exquisite molecular mechanisms to sense environmental and cellular stress, allowing them
to build proper protective and adaptive responses. This is not an
easy task, as challenges seldom come one at a time; but rather,
interconnect. For example, thermal stress promotes protein
folding defects and proteostatic stress, which, in turn, requires
substantial cellular energy to be resolved, leading to increased
energy demands. Organisms not only have to develop highly
specialized stress-sensing mechanisms and defense pathways to
overcome them but also require a hierarchical coordination of
different stress response mechanisms. Hence, several proteins
and enzymes act as stress response hubs, or “master regulators,”
integrating information on the time, frequency, and amplitude
of different stresses to mount an orchestrated defense.
The response to acute challenges often involves the activation of specialized transcriptional regulators that rewire gene
expression, switching on genes aimed to alleviate the stress
while shutting down transcriptional paths that are not needed
for the immediate survival of the cell. For example, temperatures above 42°C lead to the activation of the heat shock response in most, if not all, mammals (Anckar and Sistonen,
2011). This response is coordinated by diverse evolutionarily
conserved transcription factors, among which the heat shock
factor 1 (HSF1) is the most well-known. HSF1 activation leads
to the transcriptional up-regulation of multiple genes encoding heat shock proteins, which act as molecular chaperones
preventing and resolving protein folding defects (Anckar and
Sistonen, 2011). During the last decade, it has become clear
that HSF1 is necessary in response to changes in temperature
or proteostatic stress and also has multiple vital physiological
functions and transcriptional targets beyond heat shock proteins
(Anckar and Sistonen, 2011). Recently, HSF1 has also been
linked to metabolic control, as nutrient deprivation reduces
Correspondence to Carles Cantó: [email protected]
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HSF1 DNA binding activity through the direct phosphorylation
of HSF1 at Ser121 by the energy sensor AMP-activated protein
kinase (AMPK; Dai et al., 2015). In this issue, Qiao et al. further delved into the mechanisms by which HSF1 conjugates the
responses to proteostatic and energy stresses.
Qiao et al. (2017) started by determining how HSF1-deficient mice display a marked drop in cellular ATP and NAD+
levels. Because cellular ATP production is coupled to cellular
NAD+ levels, the authors speculated that NAD+ deficits could be
at the root of the ATP loss. In line with that, they observed that
the livers from HSF1-deficient mice showed lower levels of nicotinamide phosphoribosyltransferase (Nampt), a key enzyme
in the maintenance of intracellular NAD+ levels, by salvaging
NAD+ from nicotinamide (Cantó et al., 2015). Qiao et al. (2017)
explain this phenomenon by showing via chromatin immunoprecipitation quantitative PCR assays that HSF1 directly binds
to the Nampt promoter. Interestingly, this binding to the Nampt
promoter was higher when glucose availability was limited in
cultured hepatocytes, as well as in fasted livers, as compared
with the refed state. In contrast, they found that the binding of
HSF1 to the Hsp70 promoter was higher in the refed condition,
showing an opposite regulation to that of the Nampt promoter
and suggesting that nutrient availability signals disentangle the
metabolic transcriptional actions of HSF1 from its canonical
ones related to the heat shock response.
During the last decade, NAD+ has been recognized
not only as a redox cofactor but also as an important degradation substrate for multiple enzymes. The sirtuin family of
deac(et)ylase enzymes are critical metabolic regulators that use
NAD+ as a cosubstrate. The activity of some of its members,
such as SIRT1 and SIRT3, are rate-limited by low intracellular
NAD+ levels (Cantó et al., 2015). Therefore, Qiao et al. (2017)
tested whether sirtuin enzymes may be affected by the changes
in NAD+ levels in HSF1-deficient livers. Along this concept,
they found that SIRT1 and SIRT3 targets displayed a marked
hyperacetylation upon HSF1 deficiency, suggesting that the
enzymes are less active in HSF1-deficient cells. SIRT1 and
SIRT3 are known to play a critical role in the maintenance of
mitochondrial function, and, consistently, Qiao et al. (2017)
observed that HSF1 deficiency, either via gene ablation or
knockdown, led to decreased mitochondrial number. Interestingly, mitochondria were rounder and bigger in the livers of
HSF1-deficient mice, either in the fed or the fasted state. To
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© 2017 Cantó This article is distributed under the terms of an Attribution–Noncommercial–
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Published February 9, 2017
JCB • Volume 216 • Number 3 • 2017
Figure 1. Model of HSF1 function in oxidative metabolism. Based on
the analysis of HSF1-deficient animals and cells, the work of Qiao et al.
(2017) suggests that, under nutrient stress conditions, hepatic HSF1 activation stimulates the expression of Nampt at the transcriptional level. Elevated levels of Nampt lead to elevated levels of NAD+, which contribute to
increase sirtuin activity in the cell. As regulators of mitochondrial function,
sirtuins reprogram the cell to increase energetic efficiency and maximize
ATP regeneration, thereby ensuring energy supply to maintain key biological processes, such as protein translation and gluconeogenesis in the liver.
Figure adapted from Qiao et al. (2017).
nisms described in worms are conserved in mammals. Given
the influence of the fasting response on HSF1, it seems logical
that energy-stress mechanisms should influence its function.
Indeed, some findings already support this option: It has been
described that SIRT1, generally activated by nutrient stress,
prompts a more sustained HSF1 transcriptional activity response, probably through the direct deacetylation of Lys80
(Westerheide et al., 2009). Hence, fasting-induced SIRT1
activation in the liver could influence an increase in Nampt
expression via HSF1. HSF1 could help NAD+-consuming enzymes to ensure NAD+ supply via the transcriptional activation
of salvaging pathways. It has also been described that HSF1
knockdown leads to a transcriptional profile that overlaps with
that of the metabolic stressor metformin (Dai et al., 2015). Interestingly, metformin inhibits the respiratory complex I, also
leading to partial respiratory dysfunction, as now reported
in HSF1 knockout livers by Qiao et al. (2017). The decrease
in AMP/ATP ratio triggered by metformin is enough to activate AMPK, which in turn phosphorylates HSF1 and inhibits
HSF1 DNA binding on heat shock response elements (Dai et
al., 2015). It is puzzling that AMPK is a well-known activator
of hepatic mitochondrial biogenesis, which contradicts the observed detrimental effects on mitochondrial function triggered
by reduced HSF1 activity in hepatocytes. Certainly, HSF1 is
unlikely to be the sole mediator in the regulation of mitochondrial biogenesis by AMPK. However, the results from Qiao
et al. (2017) open another possibility, as their comparison of
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explain this observation, one possibility is that HSF1-deficient
livers accumulate damaged mitochondria. Mitochondrial fission
is a primordial event for the effective clearance of altered mitochondria through mitophagy (Toyama et al., 2016). Qiao et al.
(2017) assessed the activation of the fission regulator Drp1 via
phosphorylation analyses of Ser622, which were suggestive of
impaired fission rates upon HSF1 deficiency, explaining the accumulation of big, dysfunctional mitochondria. An alternative
explanation is that the higher fusion rates might be adaptive to
maximize energy output in situations of NAD+ crisis. Nevertheless, the accumulation of dysfunctional mitochondria leads
to impaired ATP synthesis capacity. In line with this analysis,
the researchers documented impaired metabolic processes with
high-energy demands, such as hepatic glucose production and
protein translation in HSF1 knockout mice.
If all the phenotypes of HSF1-deficient livers derive
from an NAD+ crisis, it should be expected that recovering
NAD+ levels would reverse the phenotypes of HSF1-null hepatocytes. To test this hypothesis, Qiao et al. (2017) performed
a series of elegant experiments using Nampt-independent
NAD+ precursors, such as nicotinamide riboside (NR) and
nicotinamide mononucleotide (NMN). The treatment with
either NR or NMN was enough to fully rescue intracellular
NAD+ levels, mitochondrial respiration, and ATP levels in
HSF1-deficient hepatocytes. Further, NR also improved hepatic glucose production capacity in HSF knockout mice,
based on pyruvate tolerance tests. These experiments certified
the relevance of NAD+ as the central element driving the metabolic decline caused by HSF1 deficiency. Overall, the work
of Qiao et al. (2017) shows that HSF1 depletion affects the
transcriptional regulation of Nampt levels. Lower amounts of
Nampt lead to lower amounts of NAD+, which in turn dampen
sirtuin activity and thereby mitochondrial biogenesis and oxidative capacity (Fig. 1).
It seems intuitive to think that critical processes in the
cell, such as proteostatic responses and cellular bioenergetics,
have to be exquisitely interconnected to adjust cellular energy
demands and production. This can be achieved by using similar molecular nodes, such as HSF1, and making them juggle their different functions in a single, parallel, or opposite
fashion depending on the upstream stress signals as well as
on downstream effectors. Worm models have been very useful
to understand how separated gene sets are affected by HSF1.
Ectopic expression of HSF1 not only confers resistance to protein aggregation and thermal stress but also enhances lifespan
in Caenorhabditis elegans (Hsu et al., 2003). Interestingly,
overexpression of a HSF1 protein lacking 84 amino acids in
the C-terminal side increases lifespan and thermotolerance
without affecting the transcriptional regulation of chaperone
proteins (Baird et al., 2014). Hence, the C-terminal end of
the worm HSF1 protein might act as a critical platform for
signaling enzymes or transcriptional coregulators triggering
HSF1-mediated expression of molecular chaperones. Further,
a recent model of HSF1 overexpression in the worm nervous
system has allowed complete separation of the mechanisms
by which HSF1 regulates thermotolerance and aging (Douglas et al., 2015). Importantly, the longevity effects of HSF1 in
worms were attributed to the impact of HSF1 on daf-16, an
evolutionarily conserved regulator of the metabolic response
to fasting in worms and mammals (Douglas et al., 2015). An
important future line of research is elucidating how HSF1 integrates stress and metabolic signals and whether the mecha-
Published February 9, 2017
the binding of HSF1 to the Nampt and the Hsp70 promoters
indicates that the repression of HSF1 DNA binding to heat
shock elements does not necessarily imply that the DNA binding to other genes is blunted. Indeed, phosphorylation of transcriptional regulators by AMPK does not always act as an on/
off switch, but rather fine-tunes their activity toward specific
gene sets by, for example, allowing or obstructing interaction
with other transcriptional regulators. AMPK activation is also
linked to higher Nampt expression (Fulco et al., 2008); however, the role of HSF1 in this process is not yet known.
Altogether, the involvement of HSF1 in the maintenance
of NAD+ levels makes it a central metabolic node and adds another layer of complexity to the multiple and distinct actions
of HSF1 beyond proteostatic responses. It will be interesting
to evaluate the different molecular cues and posttranscriptional
modifications that drive HSF1 toward particular gene sets and
their hierarchical nature. In this sense, it would be important to
understand the impact of HSF1 gain-of-function on mammalian
health and lifespan. As with all good scientific findings, these
new observations on HSF1 give new life to its roles in mammals
and leave a wealth of tasty questions to open up the appetite of
avid researchers on the field.
Carles Cantó is an employee of the Nestlé Institute of Health
Sciences S.A.
The author declares no further competing financial interests.
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A heat shock factor controls oxidative metabolism • Cantó
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